The recently developed methods of electron crystallography are uniquely valuable for structural studies on certain cell membranes and on thin, sheetlike crystals of proteins and other biological macromolecules. We intend to advance and extend the power and capabilities of these electron crystallographic methods by constructing a two-dimensional detector suitable for recording electron diffraction patterns in the pulse-count mode. At the present time, multiple isomorphous replace (MIR) phasing is more difficult for electrons than for x-rays, because of the smaller dependence of atomic scattering as a function of atomic number. As a result, the differences in electron diffraction intensities for native and (heavy atom) derivative structures are expected to be similar to the error inherent in photographic recording of intensities. The proposed area detector should give much greater accuracy in the measurement of diffraction intensities, however, and the improved accuracy is expected to make it possible to carry out MIR phasing of electron diffraction patterns, in the resolution range where direct phasing from images has proven to be very difficult. The work proposed is intended to demonstrate the feasibility and advantages of using a charge-coupled device as a two-dimensional area detector. If the proposed work is successful, we will use the results as the basis for designing a full experimental system that is capable of data collection on a routine basis.